1. Field of the Invention
The present invention relates to incandescent lamps, particularly of a type exhibiting longer-than-normal life expectancy.
2. Description of Prior Art
A common approach to attaining increased life expectancy of an incandescent lamp is that of reducing the amount of power supplied to its filament; which implies a reduction in the filament's operating temperature. However, reducing the filament's operating temperature results in decreased luminous efficacy; which under most circumstances is undesirable. One approach to attaining increased lamp life by reduction of filament temperature is described in U.S. Pat. No. 4,229,680 to Nilssen.
Another approach is that of enclosing the filament within an envelope operative to permit relatively easy transmission of photons of visible frequencies, while reflecting back onto the filament photons of infrared frequencies. That way, some of the heat-generating energy may be conserved, thereby making the overall production of vidible light more efficient. An improved-efficacy incandescent lamp based on this approach is described in U.S. Pat. No. 4,612,473 to Nilssen.